The present invention relates to laboratory equipment used for research mainly in the field of biology, and more particularly, to preparation-making ultracentrifuges.
The invention can be used in molecular biology, biochemistry, virology, medicine and other scientific fields for investigation of proteins, nucleic acids, for separation and concentration of viruses and in all other applications where substances of different densities in a liquid phase have to be separated in powerful centrifugal fields.
There exists a high-speed gas centrifuge used in such types of research work for separation of gaseous substances; this centrifuge comprises an evacuated housing which contains a thin-walled hollow rotor of cylindrical shape, which is driven by an electric motor disposed within the centrifuge housing. A magnetic suspension is provided to hold the rotor in a vertical position, said magnetic suspension being arranged within the centrifuge housing above the rotor. The magnetic suspension comprises a cylindrical permanent magnet and an armature secured to the upper cover of the rotor. Attached to the lower cover of the rotor is a flexible spindle by means of which the rotor rests on a hydraulically damped step bearing mounted in the rotor support. The electric motor uses a butt-type stator and a disc-shaped armature mounted on the lower cover of the rotor. The centrifuge is provided with several tubes to feed gas into the rotor and obtain the fractions. The gas centrifuge functions as follows. The electric motor, which operates as an asynchronous machine, accelerates the rotor to the required speed and then maintains this speed constant.
A gas is fed into the rotor through one of the tubes, fills the rotor and then spins together with it. After the required centrifugation time the gas fractions are removed. The rotor can be filled with gas and fractions can be obtained continuously with the centrifuge running. In view of this, no limitations are placed on the time required to accelerate the rotor to the required speed. The centrifuge does not have to meet any special requirements with regard to temperature during this operation. Partially because it is designed to operate continuously and has no apparatus for temperature control the gas centrifuge cannot be used as a preparation centrifuge by merely replacing its rotor. The design of the electric motor and of the magnetic suspension of the rotor in the gas centrifuge are largely determined by the specific design features of the rotor. The relatively small weight of the rotor makes it possible to use a permanent magnet in the magnetic suspension. The flat face of this permanent magnet is disposed above the cylindrical rotor and produces a sufficient lift for the given type of rotor without the need of any additional controllable magnetic circuit. One of the conditions on which normal operation of a thin-walled rotor of the gas centrifuge depends is the presence of low masses at the rotor ends. This condition necessitates the use of a butt-type stator in the electric motor, the armature thereof being in the form of a disc which can be mounted directly on the rotor. The rotor of a preparation centrifuge is much heavier and has different inertia parameters. Therefore without an additional magnetic circuit in the rotor magnetic suspension of the gas centrifuge it is not possible to obtain the required combination of a vertical and horizontal rigidity of the magnetic coupling required for the dynamic stability of a preparation rotor and for its reliable operation. An increase in the vertical rigidity can be obtained if the lift produced by the magnetic suspension is increased by moving its armature closer to the permanent magnet but this makes the armature liable to stick to the magnet during operation and may result in a breakdown.
In a preparation-making centrifuge the time during which the rotor has to be accelerated and braked, is, as a rule, limited to within 30 minutes.
The electric motor used in the gas centrifuge cannot accelerate the rotor to the required speed within such a short period of time as the torque it develops is rather low. The power of the electric motor cannot be increased by merely increasing its size as the increased attraction of the disc-shaped armature to the stator overloads the rotor support beyond permissible limits.
There exists a preparation-making ultracentrifuge comprising an evacuated housing which contains a vertically mounted rotor surrounded by a temperature-regulating shell and adapted for accommodation of vessels with preparations, said rotor being disposed coaxially with and below a magnetic suspension and above a rotor drive and a rotor support.
The magnetic suspension of the existing preparation-making centrifuge is disposed outside the centrifuge housing and comprises a solenoid which holds the rotor in a vertical position and consists of three sections separated from one another by brass sheets cooled by copper coils through which water is passed. Mounted inside the solenoid is a core suspended from a piano wire. The lower part of the core is located in an oil bath.
In the existing preparation-making centrifuge the conical upper end of the rotor is located within the magnetic field of the core.
The centrifuge includes an electronic servo system which holds the rotor in a vertical position by adjusting the current through the solenoid. The rotor is driven by an air turbine, the turbine drive being disposed outside the centrifuge housing. Rotation of the turbine is imparted to the rotor by means of a flexible steel shaft, passing through a vacuum seal, and a connecting stud which engages a slot made in the lower part of the rotor. When the rotor is accelerated to the required speed, the flexible shaft is disengaged from the rotor by means of a lever specially provided for the purpose. A neoprene washer mounted at the connecting stud base keeps the centrifuge housing airtight when the connecting stud moves down. The rotor of the centrifuge spins freely due to inertia when the pressure inside the centrifuge housing is about 10.sup.-6 mm Hg.
Horizontal oscillations of the rotor which often appear during operation cause displacements of the solenoid with the core, which are damped in the oil bath.
Separation of the liquid during operation of the centrifuge is monitored through a window provided in the centrifuge housing.
The existing construction of a preparation ultracentrifuge has a number of disadvantages, one of which being that the rotor drive is disposed outside the centrifuge housing. This requires the provision of vacuum seals on the shaft. The fact that the drive has to be disengaged after acceleration of the rotor makes it possible to maintain a constant rotation speed of the rotor for a long time and therefore limits the time of the experiment. The presence of vacuum seals on the rotating flexible shaft of the rotor reduces the reliability and operating life of the centrifuge. The operational reliability of the electronic servo system which serves for vertical stabilization of the rotor is not high enough.
Still another disadvantage of the existing ultra centrifuge is that the pressure within the centrifuge housing has to be maintained at about 10.sup.-6 mm Hg in order that the rotor can spin due to inertia for a long time without an appreciable drop in its speed. This necessitates the use of special vacuum pumps and seals.